The present invention relates to a control system for an internal combustion engine having a compression ratio control mechanism capable of changing an engine compression ratio.
Heretofore, for a variable control of an engine compression ratio through a variable control of a piston stroke, i.e., a volume of a combustion chamber of a reciprocative-piston internal combustion engine, the following techniques (a) to (f) are known.
(a) When engine knock is detected, an engine compression ratio is first lowered. Then, an ignition timing is delayed when engine knock is still detected as disclosed in Japanese patent publication No. 5-59273.
(b) Combustion condition control factors are controlled in accordance with a difference of a detected engine compression ratio from a target engine compression ratio as disclosed in Japanese patent publication No. 6-92746.
(c) An engine operating condition is detected to determine a target engine compression ratio, and an engine compression ratio is changed step by step from the present engine compression ratio to the target engine compression ratio as disclosed in Japanese patent publication No. 7-113332.
(d) An engine operating condition is detected to determine a target engine compression ratio, and when the difference between the present engine compression ratio and the target engine compression ratio is larger than a predetermined value, the correction width for correction of the target engine compression ratio is made larger on the compression ratio decrease side than on the compression ratio increase side as disclosed in Japanese patent publication No. 7-116956.
(e) The correction value of the engine compression ratio is determined in accordance with the frequency of engine knock and stored to execute a compression ratio control as disclosed in Japanese patent publication No. 8-33112.
(f) An engine compression ratio is decreased when a combustion pressure detected by a combustion pressure sensor increases above a first set value and increased when the combustion pressure decreases below the first set value as disclosed in Japanese patent publication No. 6-35842.
In the engine having a compression ratio control mechanism, the engine compression ratio is preferably decreased to avoid engine knock at engine acceleration at which the engine speed increases. However, the compression ratio control actuator that actuates the compression ratio control mechanism is operated at a predetermined response speed and though needless to say, the response speed has a limit irrespective of whether the actuator is of the motor-driven type or of the hydraulically driven type. Accordingly, when the engine compression ratio cannot be changed at a desired response speed in response to a variation of the vehicle speed as at fast acceleration, i.e., when the response speed of the compression control actuator is lower than a required speed, knock may possibly be caused during engine acceleration. On the other hand, when the response speed of the compression ratio control actuator is too fast as at slow acceleration, the engine compression ratio is decreased largely though the vehicle has not yet been accelerated sufficiently, thus making it impossible to obtain a desired engine torque.
In the above described techniques (a) to (f), a combustion pressure or the like corresponding to engine knock during engine acceleration is detected and on the basis of the result of detection a control for decreasing the engine compression ratio is started, or the engine operating condition is detected to determine a target engine compression ratio and the engine compression ratio is controlled so as to become closer to the target compression ratio. For this reason, the time at which decrease of the engine compression ratio is started in response to engine acceleration is liable to be delayed. Accordingly, the engine compression ratio cannot be changed at a required speed, thus causing a possibility of engine knock. Thus, it is necessary to execute a control for preventing engine knock such as retardation of the ignition timing frequently, or it is necessary to make sufficiently faster the speed at which the compression ratio is varied. When this is the case, an increase in size of the compression ratio control actuator and an increase in energy consumption are incurred. Further, in case the speed at which the engine compression ratio during engine acceleration is decreased stepwise for the purpose of making smaller a variation of engine torque due to a decrease of the engine compression ratio during engine acceleration, there is a possibility that the stepwise decreasing speed of the engine compression ratio is deviated from an ideal speed to cause engine knock. To avoid this, it is also necessary to execute retardation of the ignition timing or the like control frequently.
The present invention has been made in view of the foregoing problems and has its object to provide a control system for an internal combustion engine that can avoid engine knock during engine acceleration assuredly and can suppress a torque variation and a decrease of fuel consumption effectively.
To achieve the above object, there is provided according to an aspect of the present invention a control system for an internal combustion engine comprising a compression ratio control mechanism capable of changing an engine compression ratio, and a control unit for controlling the compression ratio control mechanism in accordance with an acceleration state of the engine, the control unit including an acceleration parameter obtaining section for obtaining an acceleration parameter corresponding to a change of an engine speed, an acceleration determining section for determining whether the engine is in a slow or fast acceleration state on the basis of the acceleration parameter, and a control section for controlling at least one of a compression ratio change speed at which the engine compression ratio during engine acceleration is changed and a compression ratio change start time at which a change of the engine compression ratio during engine acceleration is started, on the basis of whether the engine is in a slow or fast acceleration state.
According to another aspect of the present invention, there is provided a control system for an internal combustion engine comprising a compression ratio control mechanism capable of changing an engine compression ratio, acceleration parameter obtaining means for obtaining an acceleration parameter corresponding to a change of an engine speed, acceleration determining means for determining whether the engine is in a slow or fast acceleration state on the basis of the acceleration parameter, and control means for controlling at least one of a compression ratio change speed at which the engine compression ratio during engine acceleration is changed and a compression ratio change start time at which a change of the engine compression ratio during engine acceleration is started, on the basis of whether the engine is in a slow or fast acceleration state.
According to a further aspect of the present invention, there is provided A control method for an internal combustion engine having a compression ratio control mechanism capable of changing an engine compression ratio, the control method comprising obtaining an acceleration parameter corresponding to a change of an engine speed, determining whether the engine is in a slow or fast acceleration state on the basis of the acceleration parameter, and controlling at least one of a compression ratio change speed at which the engine compression ratio during engine acceleration is changed and a compression ratio change start time at which a change of the engine compression ratio during engine acceleration is started on the basis of whether the engine is in a slow or fast acceleration state.